Detonator
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A detonator, sometimes called a blasting cap in the US, is a small
Detonators come in a variety of types, depending on the way they are initiated (chemically, mechanically, or electrically) and details of their inner working, which often involve several stages. They include non-electric caps, electric caps, and fuse caps, the last two being the most common. Electric types are set off by a short burst of current sent to the cap, by a
Old detonators used
History
The first blasting cap or detonator was demonstrated in 1745 when British physician and apothecary William Watson showed that the electric spark of a friction machine could ignite black powder, by way of igniting a flammable substance mixed in with the black powder.[1]
In 1750,
In 1832, a hot wire detonator was produced by American chemist Robert Hare, although attempts along similar lines had earlier been attempted by the Italians Volta and Cavallo.[3] Hare constructed his blasting cap by passing a multistrand wire through a charge of gunpowder inside a tin tube; he had cut all but one fine strand of the multistrand wire so that the fine strand would serve as the hot bridgewire. When a strong current from a large battery (which he called a "deflagrator" or "calorimotor") was passed through the fine strand, it became incandescent and ignited the charge of gunpowder.[4][5]
In 1863,
In 1868, Henry Julius Smith of Boston introduced a cap that combined a spark gap ignitor and mercury fulminate, the first electric cap able to detonate dynamite.[8]
In 1875, Smith—and then in 1887, Perry G. Gardner of North Adams, Massachusetts—developed electric detonators that combined a hot wire detonator with mercury fulminate explosive.[9][10][11] These were the first generally modern type blasting caps. Modern caps use different explosives and separate primary and secondary explosive charges, but are generally very similar to the Gardner and Smith caps.
Smith also invented the first satisfactory portable power supply for igniting blasting caps: a high-voltage magneto that was driven by a rack and pinion, which in turn was driven by a T-handle that was pushed downwards.[12]
Electric match caps were developed in the early 1900s in Germany, and spread to the US in the 1950s when ICI International purchased Atlas Powder Co. These match caps have become the predominant world standard cap type.
Purpose
The need for detonators such as blasting caps came from the development of safer explosives, that would not explode if accidentally dropped, mishandled, or exposed to fire or electric fields. This required them to have high activation energy to detonate, and in turn made them correspondingly difficult to detonate intentionally. Then comes the detonator, with its role to provide the required activation energy with a small initiating explosion. As a small device, it is easy to safely store and handle, and, while still hazardous, would do little damage even if it accidentally fired. The detonator and the main explosive device can be kept separated, and only joined just before use, keeping the main charge safe.
Design
A detonator is usually a multi stage device, with three parts:
- at the first stage, the initiation mean (fire, electricity, etc.) provide enough energy (as heat or mechanical shock) to activate
- an easy-to-ignite primary explosive, which in turn detonates
- a small amount of a more powerful secondary explosive, directly in contact with the primary, and called "base" or "output" explosive, able to carry out the detonation through the casing of the detonator to the main explosive device to activate it.
Explosives commonly used as primary in detonators include
. Early blasting caps also used silver fulminate, but it has been replaced with cheaper and safer primary explosives. Silver azide is still used sometimes, but very rarely due to its high price.It is possible to construct a “Non Primary Explosive Detonator” (abbreviated to NPED) in which the primary explosive is replaced by a flammable but non-explosive mixture that propagates a shock wave along a tube into the secondary explosive. NPEDs are harder to accidentally trigger by shock and can avoid the use of lead.[13]
As secondary "base" or "output" explosive, you usually find
While detonators make explosive handling safer, they are hazardous to handle since, despite their small size, they contain enough explosive to injure people; untrained personnel might not recognize them as explosives or wrongly deemed them not dangerous due to their appearance and handle them without the required care.
Types
Ordinary detonators usually take the form of ignition-based explosives. While they are mainly used in commercial operations, ordinary detonators are still used in military operations. This form of detonator is most commonly initiated using a safety fuse, and used in non time-critical detonations e.g. conventional munitions disposal. Well known detonators are lead azide [Pb(N3)2], silver azide [AgN3] and mercury fulminate [Hg(ONC)2].
There are three categories of electrical detonators: instantaneous electrical detonators (IED), short period delay detonators (SPD) and long period delay detonators (LPD). SPDs are measured in milliseconds and LPDs are measured in seconds. In situations where nanosecond accuracy is required, specifically in the implosion charges in nuclear weapons, exploding-bridgewire detonators are employed. The initial shock wave is created by vaporizing a length of a thin wire by an electric discharge. A new development is a slapper detonator, which uses thin plates accelerated by an electrically exploded wire or foil to deliver the initial shock. It is in use in some modern weapons systems. A variant of this concept is used in mining operations, when the foil is exploded by a laser pulse delivered to the foil by optical fiber.
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Trimming platinum wires for use in blasting caps, 1955
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Production of blasting caps, Hercules Powder Company, 1955
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Inserting plug and bridge wire into electric blasting caps, 1955
A non-electric detonator is a shock tube detonator designed to initiate explosions, generally for the purpose of demolition of buildings and for use in the blasting of rock in mines and quarries. Instead of electric wires, a hollow plastic tube delivers the firing impulse to the detonator, making it immune to most of the hazards associated with stray electric current. It consists of a small diameter, three-layer plastic tube coated on the innermost wall with a reactive explosive compound, which, when ignited, propagates a low energy signal, similar to a dust explosion. The reaction travels at approximately 6,500 ft/s (2,000 m/s) along the length of the tubing with minimal disturbance outside of the tube. Non-electric detonators were invented by the Swedish company Nitro Nobel in the 1960s and 1970s, and launched to the demolitions market in 1973.
In civil mining, electronic detonators have a better precision for delays. Electronic detonators are designed to provide the precise control necessary to produce accurate and consistent blasting results in a variety of blasting applications in the mining, quarrying, and construction industries. Electronic detonators may be programmed in millisecond or sub-millisecond increments using a dedicated programming device.
Wireless electronic detonators are beginning to be available in the civil mining market.[14] Encrypted radio signals are used to communicate the blast signal to each detonator at the correct time. While currently expensive, wireless detonators can enable new mining techniques as multiple blasts can be loaded at once and fired in sequence without putting humans in harm's way.
A number 8 test blasting cap is one containing 2 grams of a mixture of 80 percent mercury fulminate and 20 percent potassium chlorate, or a blasting cap of equivalent strength. An equivalent strength cap comprises 0.40-0.45 grams of PETN base charge pressed in an aluminum shell with bottom thickness not to exceed to 0.03 of an inch, to a specific gravity of not less than 1.4 g/cc, and primed with standard weights of primer depending on the manufacturer.[1]
Blasting caps
The oldest and simplest type of cap, fuse caps are a metal cylinder, closed at one end. From the open end inwards, there is first an empty space into which a pyrotechnic
Solid pack electric blasting caps use a thin bridgewire in direct contact (hence solid pack) with a primary explosive, which is heated by electric current and causes the detonation of the primary explosive. That primary explosive then detonates a larger charge of secondary explosive. Some solid pack fuses incorporate a small pyrotechnic delay element, up to a few hundred milliseconds, before the cap fires.
Match type blasting caps use an electric match (insulating sheet with electrodes on both sides, a thin bridgewire soldered across the sides, all dipped in ignition and output mixes) to initiate the primary explosive, rather than direct contact between the bridgewire and the primary explosive. The match can be manufactured separately from the rest of the cap and only assembled at the end of the process. Match type caps are now the most common type found worldwide.
The
In laser ordnance initiators, a pulse from a laser passes down an optical fiber to strike and thus initiate a carbon-doped explosive. These initiators are highly reliable. Unintentional initiation is very difficult as the explosive can only be detonated by the attached laser, which is precisely tuned to do so, or a completely independent laser that matches.
See also
- Exploding-bridgewire detonator – Detonator fired by electric current
- Explosive booster – Sensitive explosive charge
- TM 31-210 Improvised Munitions Handbook – United States Army manual
- Triggering sequence– Sequence of events that results in the detonation of explosives
References
- "But I can, at pleasure, fire gunpowder, and even discharge a musket, by the power of electricity, when the gunpowder has been ground with a little camphor, or with a few drops of some inflammable chemical oil."
- ^ Franklin, Benjamin, Experiments and Observations on Electricity at Philadelphia in America (London, England: Francis Newberg, 1769), p. 92. From p. 92: "A small cartridge is filled with dry powder, hard rammed, so as to bruise some of the grains; two pointed wires are then thrust in, one at each end, the points approaching each other in the middle of the cartridge till within the distance of half an inch [1.27 cm]; then, the cartridge being placed in the circle [i.e., circuit], when the four [Leyden] jars are discharged, the electric flame leaping from the point of one wire to the point of the other, within the cartridge amongst the powder, fires it, and the explosion of the powder is at the same instant with the crack of the discharge."
- ^ "Standing Well Back - Home - Inventing detonators". www.standingwellback.com. 18 November 2012. Retrieved 22 March 2018.
- ^ Hare, Robert (1832) "Application of galvanism to the blasting of rocks," The Mechanics' Magazine, 17: 266–267.
- ^ Note: Robert Hare had constructed his large battery (or "deflagrator" or "calorimotor", as he called it) in 1821. See: Hare, R. (1821) "A memoir on some new modifications of galvanic apparatus, with observations in support of his new theory of galvanism," The American Journal of Science and Arts, 3: 105–117.
- ^ Patent for nitroglycerin: Nobel, A., British patent no. 1,813 (July 20, 1864).
- ^ See:
- Patent for dynamite: Nobel, Alfred, English patent no. 1,345 (May 7, 1867).
- Nobel, Alfred, "Improved explosive compound" Archived 2017-04-03 at the Wayback Machine, U.S. patent no. 78,317 (May 26, 1868). (See p. 2 for the description of the "percussion-cap".)
- de Mosenthal, Henry (1899) "The life-work of Alfred Nobel," Journal of the Society of Chemical Industry, 18: 443–451; see p. 444.
- ^ Smith, Henry Julius, "Improvement in electric fuses," Archived 2021-08-31 at the Wayback Machine U.S. Patent no. 79,268 (June 23, 1868).
- ^ Cooper, Paul W., Explosives Engineering (New York, New York: Wiley-VHC, 1996), p. 339.
- ^ See:
- Smith, Henry Julius, "Improvement in electric fuses," Archived 2017-04-04 at the Wayback Machine U.S. Patent no. 173,681 (February 15, 1876).
- Smith, Henry Julius, "Electric fuse," Archived 2021-08-31 at the Wayback Machine U.S. Patent no. 225,173 (March 2, 1880).
- ^ Gardner, Perry G., "Electric fuse," Archived 2017-04-04 at the Wayback Machine U.S. Patent no. 377,851 (February 14, 1888).
- ^ See:
- Smith, Henry Julius, "Improvement in magneto-electric machines," Archived 2022-05-05 at the Wayback Machine U.S. Patent no. 201,296 (January 17, 1878).
- Smith, Henry Julius, "Dynamo-electric igniting machine," Archived 2022-05-05 at the Wayback Machine U.S. Patent no. 353,827 (December 7, 1886).
- Smith, Henry Julius, "Art of blasting," Archived 2022-05-05 at the Wayback Machine U.S. Patent no. 534,289 (February 19, 1895).
- Krehl, Peter O. K., History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference (Berlin, Germany: Springer, 2009), p. 365.
- ^ ’’Safety Data Sheet: Exel Neo (1.4S)’’. Orica UK Limited. 11/09/2023 rev. 1.0.
- ^ "Improving safety and productivity". www.oricaminingservices.com. Retrieved 2019-05-16.
Further reading
- Cooper, Paul W. Explosives Engineering. New York: Wiley-VCH, 1996. ISBN 0-471-18636-8.